This invention generally relates to nuclear reactors which are cooled by a liquid metal and, more particularly, to those nuclear reactors using dip-ring seals to isolate the liquid metal coolant from the atmosphere.
Most breeder reactors that are cooled by a liquid metal have some form of dip-ring seal. A dip-ring seal consists of a deep trough filled with a heavy metal and skirt ring that is dipped into the trough. The heavy metal has a low melting temperature and when heated, the seal provides a fluid barrier between a rotating shield plug and the support structure of the reactor. A rotating shield plus is a massive circular disc that forms the top cover of a reactor. The dip-ring seal permits the rotating plug to move while maintaining a position barrier between the liquid metal coolant and the atmosphere. Typically two or three rotating plugs are used in each reactor in order to permit refueling and fuel handling operations inside of the reactor and underneath the shield plugs.
Heretofore, dip-ring seals and the heavy metal in the trough have been subject to contamination. This contamination originates from the liquid metal vapor in the reactor, usually sodium, which diffuses up the annulus formed between each rotating plug and its support structure. The liquid metal vapor diffuses into the heavy metal in the dip-seal trough and causes contamination. Secondly, when the heavy metal in the dip-seal trough has been heated and melted for rotation of the shield plug, the oxygen in the atmosphere reacts with the molten metal and causes an oxide film to form. Over the years both the sodium and the oxygen cause a percipitation to form which agglomerates into a viscous dross. This dross eventually impedes rotation of the shield plugs.
A further difficulty with prior dip-ring seal designs has been the physical inaccessability of the troughs. In prior designs the dip-ring seals were located far down in the annulus between the shield plug and its support structure. The dip-seals were almost inaccessable for maintenance, and when the viscous dross was formed, removal of the contaminated heavy metal from the trough was very difficult.